Margaret Thatcher, John Major and William Hague could all attest to the truth of Winston Churchill's famous dictum on the House of Commons chamber: the opposition occupies the benches in front of you, but the enemy sits behind you.

Iain Duncan Smith will certainly have enemies to deal with in his own deeply divided party.

He will also have to contend with the political gameplans adopted by Labour and Liberal Democrats in response to his election as Conservative leader.

Labour, admittedly from the luxury of its commanding position - mega-majority in parliament, consistently ahead in the polls, facing a weak and deeply unpopular opposition - was in a win-win situation.

Whoever emerged to succeed William Hague would have to face those brute facts of political life for today's Tory Party.

Charles Kennedy: Will be looking for defectors

But many Labour MPs and ministers believe Mr Duncan Smith's victory has as near as possible guaranteed them the third term they were already confident of winning.

As a mark of respect in the wake of the terrorist attacks on the US, normal political argy-bargy has been suspended - hence the Conservatives postponed unveiling their new leader until Thursday, and Labour declined to issue any formal comment on the result.

But the strategy Labour will adopt once normal service is resumed is clear: to paint the party as increasingly extreme, euro-obsessed, backward-looking - and utterly divided.

Incriminating quotes

Mr Duncan Smith's closeness to Baroness Thatcher will be made great play of.

Utterances issued by various parties in the heat of the campaign will have been closely noted, logged and filed in Labour's famous Excalibur database.

Expect to be reminded of incriminating quotes such as Archie Norman's complaint that too many in his party were "in the thrall" of Lady Thatcher.

"They adhere to a distilled reinterpretation of Thatcherism that has become an obstacle to new thinking," he went on.

Expect to hear replayed the soundbites from John Major's radio interview in which he angrily questioned the "very right-wing" Mr Duncan Smith's ability to command loyalty when he "night after night" voted with Labour against a Tory government fighting for survival when the Maastricht Bill was going through parliament in 1993 and 1994.

Douglas (now Lord) Hurd's comments at the weekend that if Mr Duncan Smith became leader he could in no way expect "automatic loyalty" from pro-Europeans will also doubtless be frequently revisited.

And the seriously embarrassing discovery during the contest that a vice-chairman of the future leader's campaign had close links to the far-right British National Party will be remembered too.

Lib Dems seek turncoats

As for the Liberal Democrats, even before the result leader Charles Kennedy dismissed the Tories as incapable of being led whoever won the job.

But now that the deeply Eurosceptic, never-to-the-euro Mr Duncan Smith has won it, the party intends to go into overdrive in seeking One Nation, pro-European moderate Tories to defect to the third party that cheekily styles itself the "effective opposition".

Expect Mr Kennedy to use his party conference speech next week to issue an explicit appeal to unhappy Conservatives to come over to his party instead.